Fuel cells are galvanic systems which operate following similar electro-chemical principles as in conventional storage batteries. There is a positive and negative electrode separated by an ion-conducting electrolyte adapted to carry current generated by a catalyzed chemical reaction. The fuel cell, however, has a theoretically infinite power output capability, as long as fuel and oxidant are continuously fed to the system for reaction. For example, the current flow in the traditional hydrogen-oxygen fuel cell is generally provided by the flow of electrons associated with the passage of an ion through an intervening electrolyte medium.
There are generally three distinct types of low temperature hydrogen-oxygen fuel cells: the solid polymer proton exchange membrane fuel cell, the alkaline fuel cell, and the phosphoric acid fuel cell. All of these types generally operate at below about 250.degree. C., in aqueous systems. Electrical energy is produced by the catalyzed reaction between hydrogen and an oxidizing gas, usually pure oxygen, with the movement of an ion, i.e., a proton or hydroxyl ion (OH.sup.-), through an electrolyte connecting the positive to the negative electrode. In the alkaline fuel cell, the electrolyte is highly concentrated (at least about 30 wt. %) aqueous potassium hydroxide solution, the concentration determining the maximum operating temperature. This hydroxide electrolyte is generally maintained within a solid matrix, including, for example, asbestos, together with a catalyst. The catalyst can be, in addition to the noble metals, nickel, silver, certain metal oxides and spinels.
The second type of low temperature fuel cell is the phosphoric acid fuel cell, which utilizes concentrated phosphoric acid as the electrolyte. This fuel cell operates at temperatures in the range of between 150.degree. C. and just over 220.degree. C. The concentrated acid electrolyte is preferably at approximately 100% concentration, and is retained in a solid matrix, such as silicon carbide(SiC). The electro-catalyst, which impregnates both the anode and the cathode, can be platinum or other such noble metals.
An efficient low temperature system, which also operates at temperatures below the boiling point of water, includes a solid polymeric proton exchange membrane between the fuel cell electrodes. The membrane is formed from, for example, perfluorocarbon materials sold, for example, under the trademark "NAFION".RTM. by E. I. DuPont De Nemours. A noble metal catalyst is also required for most polymeric membrane type of fuel cells.
Commonly available solid polymer electrolyte fuel cells require input of reactant gases, usually a hydrogen fuel and an oxidant, generally oxygen or air, and of water, for cooling and for maintaining the electrolyte membrane.
The cooling systems for the solid polymer electrolyte fuel cells are of two types: the water flow, or pass-through, type, where cooling water from outside the cell is provided for indirect heat exchange from impervious conduits within the cell; and the passive, or wicking, type of cell, by evaporative cooling, wherein water is caused to evaporate from the anode support plates, which are formed to have a large surface area.
For both types of cooling systems, the solid polymer electrolyte membrane must be kept moistened with water; otherwise the membrane will dry out, and become inefficient in operation as well as structurally weakened. Water is generally carried from the fuel, or hydrogen, side of the membrane, together with the proton, through the membrane, thereby tending to dry the anode side of the membrane, and causing cracking of the membrane. In operation, additional water must thus be supplied with the hydrogen, to compensate for the water removed.
One system to improve cooling of the fuel cell, while at the same time maintaining humidification of the fuel side of the membrane, is shown, for example, in U.S. Pat. No. 4,649,091.
As commercially available, the so-called "fuel cell" is actually a stacked configuration of a plurality of cells each having an anode and a cathode, with a solid electrolyte membrane between them, and passages for fuel and oxidant gas. To maintain a continuing operation of such a stack of cells requires a system that provides sufficient cooling to prevent overheating of the system and means to provide the fuel and the oxidant, in a managed system to maintain a sufficiently long operating time between shutdowns.
In some conventional fuel cell stacks, the hydrogen and oxygen gases are delivered to the stack in excess of that needed to support the electro-chemical reaction. There is thus a continuous flow through the stack, and an exhaust from the stack by which product water is removed and any inert gases are vented together with the excess hydrogen and oxygen. Generally, the great majority of the product water is removed with the oxygen purge, whereas only a relatively small amount of condensation is removed along with the hydrogen purge. Generally, the fuel and oxidant gases first pass through humidification cells within the cell stack. The gases are there saturated with pure water vapor in order to prevent dehydration of the ion-conducting membrane. The humidified gases are then passed through the anode and cathode chambers, respectively, of the cells within the stack, the cells being fed in parallel; and the excess gases are then vented from the final cell. Although the gas and liquid flow through the stack system is in parallel, i.e., through the individual cells, the electrical connection between the individual cells is conventionally in series.
The cooling water within the stack must be extremely pure, e.g., deionized water having a high resistivity. The cooling water passes through an external indirect heat exchanger where the heat is transferred to, for example, a parallel or countercurrently flowing stream of raw water. This same internal cell cooling water has been conventionally used to humidify the gas streams within the humidification stage of the cell stack.